1. Field of the Invention
The present invention relates to an organic photoconductive material, an electrophotographic photoreceptor comprising the same, and an image-forming apparatus.
2. Description of the Related Art
Recently, organic photoconductive materials have been widely researched and developed, and they are not only utilized in electrophotographic photoreceptors (hereinafter the term may be simply referred to as “photoreceptors”) but also are being applied to electrostatic recording apparatuses, sensor materials, organic electroluminescent (EL) elements, etc. In addition, electrophotographic photoreceptors that comprise organic photoconductive material are utilized not only in the field of copiers but also in other fields of printing plate materials, slide films and microfilms for which photographic technology has heretofore been used, and they are further applied to high-speed printers having a light source of lasers, light emitting diodes (LED) or cathode ray tubes (CRT). Accordingly, the demand for such organic photoconductive materials and electrophotographic photoreceptors comprising the material is increasing highly and widely.
Heretofore, inorganic photoreceptors have been widely used for electrophotographic photoreceptors, in which the photosensitive layer comprises, as the essential ingredient, an inorganic photoconductive material such as selenium, zinc oxide or cadmium. Though having the basic characteristics for themselves in some degree, inorganic photoreceptors are problematic in that films for the photosensitive layer are difficult to form and are poorly plasticized and their production costs are high. In general, in addition, inorganic photoconductive materials are highly toxic and are significantly limited in point of their production and treatment.
As opposed to these, organic photoreceptors that comprise organic photoconductive material have various advantages in that films for the photosensitive layer thereof are easy to form, and are flexible, lightweight and transparent and those of good sensitivity to wavelengths in a broad range are readily planned through suitable sensitization. Accordingly, such organic photoreceptors are being developed as the mainstream of electrophotographic photoreceptors. In the initial stage thereof, organic photoreceptors have some defects in point of their sensitivity and durability, but such defects have now been significantly overcome by the development of function-separated electrophotographic photoreceptors of which the charge-generating function and the charge-transporting function thereof are separately attained by different substances. Such function-separated photoreceptors have broad latitude in selecting the materials for the charge-generating substances that participate in the charge-generating function thereof and the charge-transporting substances that participate in the charge-transporting function thereof, and have the advantage in that those having any desired characteristics are relatively readily produced.
A variety of substances have heretofore been investigated for the charge-generating substances that may be used in the function-separated photoreceptors, including, for example, phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes and pyrylium salt dyes, and various materials of good light fastness and good charge-generating ability have been proposed.
On the other hand, various compounds are known for the charge-transporting substances, including, for example, pyrazoline compounds (e.g., those in Japanese Examined Patent Publication JP-B2 52-4188 (1977)), hydrazone compounds (e.g., those in Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), Japanese Unexamined Patent Publication JP-A 55-52063 (1980), triphenylamine compounds (e.g., those in Japanese Examined Patent Publication JP-B2 58-32372 (1983) and Japanese Unexamined Patent Publication JP-A 2-190862 (1990)) and stilbene compounds (e.g., those in Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)). Recently, pyrene derivatives, naphthalene derivatives and terphenyl derivatives that have a condensed polycyclic hydrocarbon structure as the center nucleus have been developed (e.g., those in Japanese Unexamined Patent Publication JP-A 7-48324 (1995)).
The charge-transporting substances must satisfy the following requirements:    (1) they are stable to light and heat;    (2) they are stable to ozone, nitrogen oxides (NOx) and nitric acid that may be generated in corona discharging on the surface of photoreceptors;    (3) they have good charge-transporting ability;    (4) they are compatible with organic solvents and binders;    (5) they are easy to produce and are inexpensive. Though partly satisfying some of these, however, the charge-transporting substances mentioned above could not satisfy all of these at high level.
Of the requirements mentioned above, the good charge-transporting ability is especially important for the substances. For example, in order to use a charge transportation layer that is formed by dispersing a charge-transporting substance along with a binder resin, as the surface layer of a photoreceptor, the charge-transporting substance must have good charge-transporting ability for ensuring sufficient responsiveness of the layer to light. Through the use of a photoreceptor mounted on copiers or laser beam printers, a surface layer of the photoreceptor is subject to partly cutting off by a contact part such as a cleaning blade or a charge roller that is in kept contact therewith. For enhancing the durability of copiers and laser beam printers, the surface layer of the photoreceptor must be tough to the contact part, or that is, it should be hardly cut away by the contact part and have good printing durability. For strengthening the surface layer to improve the durability of the machines, when the binder resin content of the surface layer, charge transportation layer is increased, then the responsiveness thereof to light lowers. This is because, since the charge-transporting ability of the charge-transporting substance in the charge transportation layer is low and since the charge-transporting substance in the layer is diluted to a higher degree with the increase in the binder resin content of the layer, the charge-transporting ability of the charge transportation layer is further lowered and the responsiveness thereof to light is therefore lowered. In case where the responsiveness of the layer to light is low, the residual potential thereof increases and, as a result, the photoreceptor shall be used repeatedly while the surface potential thereof is not sufficiently attenuated. In that condition, the surface charge in the part that shall be erased through exposure to light could not be fully erased, and it causes a drawback in that the image quality worsens in early stages. Accordingly, for ensuring good responsiveness of the surface layer to light, it is desired that the charge-transporting substance to be in the layer shall have good charge-transporting ability.
The recent tendency in the art of electrophotographic apparatuses such as digital copiers and printers is toward down-sizing and high-speed operability, and photoreceptors for these apparatuses are desired to have higher sensitivity enough for such high-speed operation. Accordingly, charge-transporting substances for these apparatuses are desired to have much better charge-transporting ability. In a high-speed process, the time from exposure to light to development is short, and photoreceptors of good responsiveness to light are desired. As so mentioned hereinabove, the responsiveness to light of charge-transporting substances depends on the charge-transporting ability thereof. From this viewpoint, therefore, charge-transporting substances having better charge-transporting ability are desired.
For the charge-transporting substances that satisfy the requirement, proposed are enamine compounds having higher charge mobility than that of the charge-transporting substances mentioned above (e.g., those in Japanese Unexamined Patent Publications JP-A 2-51162 (1990), JP-A 6-43674 (1994) and JP-A 10-69107 (1998)).
Also proposed is a photoreceptor which has been made to have an increased charge-transporting ability by adding thereto a polysilane, and has been improved in point of the chargeability and the film strength thereof by further adding thereto an enamine compound having a specific structure (Japanese Unexamined Patent Publication JP-A 7-134430 (1995)).
However, the properties of the photoreceptors that comprise the enamine compound described in JP-A 2-51162, JP-A 6-43674 or JP-A 10-69107 are still unsatisfactory.
Containing a polysilane, the photoreceptors described in JP-A 7-134430 are problematic in that they are too much sensitive to light and, when exposed to light during their maintenance, their photoreceptive properties are worsened.
Regarding the characteristics of photoreceptors, it is desired that their sensitivity does not lower even though they are driven at low temperatures and the change of their characteristics is small in any different conditions, or that is, their reliability is high in every condition. However, no one has heretofore succeeded in obtaining such charge-transporting substances that realize the characteristics.